In a highly publicized paper that topped yesterday’s news, a research group led by Michaël Gillon from the Université de Liège in Belgium reported the discovery of seven terrestrial planets in one solar system. It reminds me of the fairy tale about the brave tailor who killed seven flies with one strike, which made him an instant hero. But the announcement of seven planets is no fairy tale.
The discovery was made in the TRAPPIST-1 star system (named after the telescope that the scientists used), which has a low-luminosity red dwarf star at its center. TRAPPIST-1 has only about eight percent of the mass of our sun—making it just a bit bigger than Jupiter—and is nearly 40 light years from Earth. At first it seemed odd that a star that small could keep seven Earth-size planets in its gravitational control, and at such an extremely close orbital distance. Six of the planets have orbital periods ranging between 1.5 and 12.5 days, which explains why Gillon and his colleagues were able to detect them multiple times with the TRAPPIST telescope.
So this is not your ordinary solar system. It may be more comparable to Jupiter and its four Galilean moons Io, Europa, Ganymede, and Callisto. And, just as those moons are tidally locked—meaning they always show the same face toward Jupiter—so too, should be the planets orbiting TRAPPIST-1. They might be Earth-size, but they’re probably not Earth-like. As I’ve pointed out before, a lot of planetary and astronomical parameters have to be met before we can call a planet Earth-like, which would suggest the presence of life. Being tidally locked would make the presence of life extremely challenging, if not impossible, as one side of the planet facing the star would always be very hot, and the other facing away from the star would be extremely cold, no matter whether the average temperature is temperate or not. Sorry, these planets are not likely abodes of life.
The highest chance of finding life may be on the outermost planet, because it is not likely to be tidally locked. But, being farther from its very dim star, it might be too cold. If it has enough internal energy, there is the possibility of a subsurface ocean like Jupiter’s moon Europa, which is a high-priority astrobiology target in our Solar System.
We will likely hear much more about TRAPPIST-1 in the near future, because it’s easier to observe this system than most other exoplanets. Even if it is not the most likely place to find life, it shows us again how many surprises are out there in the Universe—solar systems very different from ours, planets we wouldn’t think are possible, and maybe types of life we haven’t even dared to dream of.